Unused content snaps not autoremoved

$ snap connections --all | grep gnome
content - gnome-3-28-1804:gnome-3-28-1804 -
content - gnome-3-34-1804:gnome-3-34-1804 -
content[gnome-3-38-2004] chromium:gnome-3-38-2004 gnome-3-38-2004:gnome-3-38-2004 -
content[gnome-3-38-2004] firefox:gnome-3-38-2004 gnome-3-38-2004:gnome-3-38-2004 -
content[gnome-42-2204] mattermost-desktop:gnome-42-2204 gnome-42-2204:gnome-42-2204 -
content[gnome-42-2204] snap-store:gnome-42-2204 gnome-42-2204:gnome-42-2204 -
content[gnome-42-2204] snapd-desktop-integration:gnome-42-2204 gnome-42-2204:gnome-42-2204 -

my system has 4 gnome-* content snaps, that were pulled in as dependencies. The apps that used them, have moved on to newer versions. Something on my system must clean then up for me, for example apt autoremoves automatically installed packages & obsolete kernels, and so should also happen with snaps.

It can be done by snapd itself, or by something else on the classic desktop - i.e. update-manager. Note it is easy to detect such snaps, as it provides no apps; has content interface only; which is not plugged by anything. If it is ever needed by any future or past revision of any other snap it would be autoinstalled back.

On my system they take up 824M of disk space (2 revisions, of 2 unused content snaps = 4 snaps)